This invention relates to concrete platforms of the type used for offshore drilling or similar work and, in particular, is concerned with a platform structure and a method for erecting it in areas where there is a weak or soft sea floor layer as is encountered in regions of high earthquake activity.
It is generally acknowledged that concrete platforms are superior to conventional steel platforms in terms of safety and economy in sites where the depth of water is greater and environmental conditions are severe. Such concrete platforms have been developed for practical use and are employed in drilling work for petroleum and natural gas in offshore installations. The nature of concrete platforms is described in my earlier U.S. patents which are incorporated herein by reference.
Concrete platforms must be capable of sustaining horizontal forces imposed upon them by wave action, tides, currents and the like. A conventional gravity type concrete platform withstands such horizontal forces by friction between its base and the sea floor. To withstand greater horizontal forces, this friction has been increased by increasing the in-water weight of the platform and this, in turn, increases the load on the sea floor, requiring the sea floor layer to possess greater supporting strength. Such increased supporting strength cannot be provided by the relatively soft or weak sea floors of the littoral seas around Japan and elsewhere.
Prominent external horizontal forces are those produced by earthquakes. These forces can be several times as great as the maximum horizontal forces imposed by sea waves. For this reason, it is desirable in a relatively lightweight platform constructed according to this invention, to disjoin structurally the caissons from the superstructure above said caissons to permit relative horizontal displacement between the caissons and the remainder of the platform during an earthquake, thereby minimizing response acceleration of the superstructure and reducing the active forces on the platform structure.